Pre-Mt. Simon Seismic Sequences Below West-Central Indiana: Local Interpretation and Regional Significance

Constraining the composition, structure, and origin of basement provinces, deep assemblages of Precambrian rocks, is largely dependent on deep boreholes and geophysical techniques. This is especially true for the eastern U.S. midcontinent. Here, I employ regional 2-D seismic reflection, Bouguer gravity, and aeromagnetic data to interpret the upper crust below west-central Indiana. Seismic reflection data were donated to Wright State University in 2015. Geopotential data are available through the USGS and affiliates. These geophysical data, together, are analyzed in a regional geologic context. Three distinct seismic stratigraphic sequences are observed on 2-D sections. The first, uppermost sequence, typified by continuous, high-amplitude, stratified reflections is constrained by boreholes and previous seismic investigations as the Paleozoic sedimentary sequence that masks the midcontinent basement. The Cambrian Mt. Simon Sandstone constitutes the base of this unit, which is underlain by the second, poorly reflective, westward-thinning sequence. Weak internal reflections create an apparent angular unconformity with the base of the Mt. Simon and appear concordant with reflections of the basal seismic package. This unit, termed the Wilbur sequence, compares well with the seismic character of the Middle Run Formation of western Ohio and Kentucky. A third, well-reflective sequence is observed at the base of the record. Stratal geometries, such as onlap and stratigraphic terminations, are locally observable on regional east-west profiles. A positive Bouguer anomaly appears associated with the apparent structural closure of this sequence, herein termed the Quincy, below northeast Owen County. Geophysical signatures of the Quincy suggest a depositional origin, composed of low-magnetic igneous rocks (rhyolites) sourced from midcontinent volcanic centers and clastic sediments from collapsed calderas. These data facilitate two alternative hypotheses. The pattern seen in west-central Indiana (Wilbur over Quincy) may correlate with that seen below southwest Ohio (Middle Run over reflective basement). This would suggest that the Middle Run was deposited over a larger regional area than early syn-rift models suggested, fortifying recent interpretations that the Middle Run represents the foreland basin sedimentary package generated by the Grenville Orogeny. It is possible, conversely, that both Wilbur and Quincy sequences are sedimentary and/or volcanic units associated with the Eastern Granite-Rhyolite Province and pre-date the Middle Run entirely. Both models suggest contrasting yet significant tectonic settings of the pre-Mt. Simon U.S. midcontinent